Touring - Sleeping bag for mountains?

Hey all,
I'm riding from Vancouver to Edmonton, starting in late May (and across the rest of Canada after that) but I'm not sure what kind of sleeping bag to get for those Western nights. I was walking around MEC here in Ottawa, looking at sleeping bags and realized I didn't really have a sense of what to expect out West at night. I own gear for nights down to -30 but I think that might be overkill, so what kind of rating do you all think should be good?

I know it can get cold up there during the day, so I'm bringing cold-weather gear which I'll send home once it is sure to stay warmer out. I could also send home my sleeping bag later too and pick up a really really light one instead (I'll probably end up doing that anyway because I hate the heat), so I don't necessarily have to compromise between the mountain cold and Eastern heat either.

I want something warm enough that I won't have to worry about not being warm enough wrapped up in my sleeping bag, cold clothing and emergency mylar blanket. ;) I have a Thermarest mattress and will be sleeping in a 1-2 person, 3-season tent.

Thanks!

Late May a 20 degree bag would be suitable for me, barring a late severe storm/odd cold snap.

There are a bunch of ways to bump the temperature rating of bags.

1. Give a look at a vapor barrier liner. They are just a bag liner which does not breeth. It makes a "micro climate" inside, making your sweat not go throught the insulation and out of the bag. Some people don't like that they leave you clamy. Don't wear all your clothes inside the VBL. Thinweight longjons&top are all that is needed (if any). If you need more insulation, stuff clothing in your bag between the VBL and bag.

2. Wear your hat & mittens. Add other clothes as needed.

3. Take a look at the Finbar Sleep Hood (http://www2.smumn.edu/deptpages/~finnovations/Hood/hood.html). I have not trully field tested it, as I haven't had it long enough. It will increase your comfort rating 15 to 20 degrees (I'd imagine) at a 6 to 7 oz weight penalty. It's made by a Christian Brother, who is a very experienced outdoorsman. Note that he's a little hard of hearing, but very nice to chat with should you call in an order. I recieved mine 2 days after I placed my order.

I'm doing a similar journey and have purchased a double thickness (http://www.mec.ca/Products/product_detail.jsp?PRODUCT%3C%3Eprd_id=845524442096075&FOLDER%3C%3Efolder_id=2534374302865701&bmUID=1144779262087)bag from MEC. It's thick on one side for cold times and thin on the other for warmer moments.

Should do the trick.

I'm in the same boat, and recently after checking out the western weather warning (http://www.bikeforums.net/showthread.php?t=186671) thread I realized that It may be a little cooler then I had expected in the mountains. I have a -12C (http://www.mec.ca/Products/product_detail.jsp?PRODUCT%3C%3Eprd_id=845524442279987&FOLDER%3C%3Efolder_id=2534374302701483&bmUID=1144783425812) bag that I got from MEC and I'm hoping that will suffice. If not, then I'll just do as stated above and wear more clothes in the sack.

Camel thanks for that link to the Finbar Sleep Hood. What an awesome piece of kit!